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Astronomers will find a dying remnant from a star cluster that “should not have existed”



Artists-impression-Phoenix-cluster-James-Joséphidès-Swinburne-astronomy-a

Left: The ball cluster tore and spread around the Milky Way. Right: Red giants that have helped astronomers understand the cluster.

James Josephides / Swinburne Astronomy

Sometime in the distant past, a starburst whipped across the Milky Way and the huge gravitational effects of the galaxy tore them apart. Gravity has formed a clot into a “large piece of spaghetti,” full of stars that are constantly flowing around our home galaxy.

New research looking at this stream of stars known as spaghetti-like Phoenix has shown that its origin is very unusual.

The study, published in the journal Nature on Wednesday, is part of a project to study stellar currents such as Phoenix in the Milky Way and is known as the “Southern Stellar Stream Spectroscopic Survey.” The survey allowed the research team to focus on Phoenix, which was discovered in 2016 during a dark energy survey that scanned the sky from 2013 to 2019.

Phoenix was once a nice, clean ball of stars held together by gravity … until it got too close to the Milky Way and was torn.

“Phoenix is ​​a long, thin stream. It’s 27,000 light-years, but it’s only 150 light-years long, ”said Geraint Lewis, an astronomer at the University of Sydney and one of the study’s authors. “It’s a large piece of spaghetti, which is a clear sign that the galaxy has completely torn it apart.”

Lewis explains that the current comes from a spherical cluster – huge spherical collections of 100,000 to several million stars that orbit the Milky Way in an area of ​​space known as the “star halo.” The Milky Way hosts about 150 of these clusters and are well known to astronomers.

But the globular cluster, which was torn billions of years ago to form the Phoenix Creek, is strange.

“This current comes from a cluster that we understand should not have existed,” Daniel Zucker, an astronomer at Macquarie University in Australia, said in a press release.

The team examined the spectrum of dozens of bright red gigantic stars contained in Phoenix to understand their chemistry and decode which elements make them up. Most of the spherical clusters that scientists know contain elements heavier than hydrogen and helium, which increases their “forging.” Astronomers studying clusters actually believe that they have a “metallurgy bottom” and that clusters cannot form below this point.

However, Phoenix is ​​made up of stars with low metallicity, which means that the cluster from which it comes decomposes under the floor of metallicity.

“Its chemical enrichment is significantly lower than all other spherical clusters that exist,” says Lewis.

This suggests an ancient origin for a spherical cluster, when the universe was quite young, and points to the kind of environment in which a spherical cluster could have formed, one that no longer exists. We caught Phoenix at the end of his life, trapped by the gravity of the Milky Way into the thin stream of spaghetti we see today.

“If we could look at the Milky Way halo billions of years ago, we’d see there were more objects than Phoenix,” Lewis explains. “Phoenix is ​​the last of its kind.”

What fate awaits the current? Eventually he disappears. The ever-present effects of gravity will begin to disperse the high concentration of stars and will fade into the stellar hall. But, Lewis says, it takes several hundred million billions years to make another Milky Way orbit. So Phoenix will hang for a while and then, after another orbit or two, it will be gone.

If we looked only a billion years later, we would never know it even existed.


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